I've just had the same issue trolling through documentation but got there!

Refer to https://developer.android.com/google/play/billing/getting-ready#configure-rtdn for googles guide on this.

It will guide you how:

• Create the pub/sub

• Configure play store to publish events to the topic

• Confirm the subscription is getting the events

You can then create a subscription that suits your needs. Eg pushes the messages to a lambda or provides a subscription for your backend server to listen to.